1. Field of the Invention
The present invention relates to electronic circuit arrangements. More particularly, this invention pertains to a novel circuit for an optoelectronic positioning tap.
2. Description of the Prior Art
Optoelectronic positioning tap arrangements find extensive application in accelerometers and gyroscopes. A known type of circuit of this type is described in DE 37 20 294 Cl and illustrated in FIG. 1.
The circuit of FIG. 1 includes a pair of closely-spaced (approximately 25 .mu.m) light-sensitive diodes D1 and D2. The diodes D1 and D2 are preferably located on the same chip and a controllable light flux is applied thereto that is generated by a light source such as a light-emitting diode (LED). Under the actuation of the light flux, the diodes supply oppositely-directed currents I1 and I2 that flow through load resistors R1, R2.
The current I1 is a function of the brightness H of the light flux emitted by the light source LED. I1 is compared with a constant current I.sub.s at a node S1 and the resultant current difference .DELTA.I.sub.s =I.sub.s -I1 is then applied to an integrating amplifier V2 that generates a control voltage U.sub.St =1/C.intg..DELTA.I.sub.s dt. The brightness H is thus corrected via a transistor T so that I.sub.1 is equalized to I.sub.s.
In the event that I.sub.1 is less than I.sub.s at the node S1, the control voltage U.sub.St at the integrating amplifier V2 will decrease (greater negative value) and the brightness H of the light flux will increase until I.sub.1 equals I.sub.s. Thus, a state of equilibrium always occurs at the node S1 independent of temperature, component variation or other asymmetries. That is, I.sub.1 equals the constant value of I.sub.s.
The diodes D1 and D2 are connected in a differential arrangement and generally integrated on a single chip, exhibiting identical properties so that I.sub.2 equals I.sub.1. At a further node S.sub.2, the difference .DELTA.I.sub.D =I.sub.2 -I.sub.k is formed that is driven to zero with uniform irradiation of the diodes D1 and D2. Since no current .DELTA.I.sub.D flows, the output voltage U.sub.A =.DELTA.I.sub.D .multidot.R.sub.A is also zero.
When a shading element such as a so-called shadow rod St, that symmetrically covers part of the light receiver areas of the diodes D1 and D2, is located in the light flux H (i.e. between the light source LED and the diodes D1 and D2) the currents I.sub.1 and I.sub.2 will remain constant despite the fact that the active diode areas have become smaller since the brightness H is corrected to exceed the integrating amplifier V2.
If the shading element is deflected, the active area of the diode D1 is reduced and that of the diode D2 increased (or vice versa). Accordingly, a current difference .DELTA.I.sub.D =I.sub.2 -I.sub.k is produced at the node S2, that generates the output voltage U.sub.A via a resistor R.sub.A of the current/voltage converting amplifier V1. I.sub.1 is maintained constant as before. That is, I.sub.1 equals I.sub.s, a constant value.
The prior art controlling circuit arrangement makes it possible largely to eliminate temperature influences, component variations and production tolerances in the positioning tap. However, the circuit according to FIG. 1 includes some fundamental shortcomings and other that lead to error influences in the circuit implementation. The most significant weaknesses of the circuit of FIG. 1 include:
(1) Only the signal of the diode D2 is evaluated to obtain the voltage U.sub.A that is positionally proportional to the shadow rod St. The signal of the diode D1 is not considered and therefore only half of the sensitivity of the optical position tap is utilized.
(2) The reaction of the current/voltage converter and amplifier V2 to the output voltage U.sub.A is not considered. The offset voltage and offset voltage drift of the real amplifier V.sub.2, however, can falsify the output voltage U.sub.A and lead to zero errors and zero error drift. (This can be verified by the following: The amplifier V2 is coupled via the resistors R1 and R2 to provide a gain in accordance with the ratio of-R.sub.A /(R1+R2). Since R.sub.A greatly exceeds (R1+R2)to achieve a high voltage gradient of U.sub.A, this error influence can be considerable. The use of the prior art circuit in, for example, highly accurate gyroscopes and accelerometers, is not possible or is greatly restricted.)
(3) The load resistors R1 and R2 load the diodes D1 and D2. Since only the short circuit current of a photodiode is strictly proportional to the intensity of illumination H, the load resistors R1 and R2 must be equal to zero to achieve the best linearity. Accordingly, the prior art circuit of FIG. 1 is not operable to achieve a pure short circuit current measurement since R.sub.1 =R.sub.2 =0 would mean an infinitely large error coupling from the amplifier V2 to the output voltage U.sub.A .multidot.(-R.sub.A /(R.sub.1 /+R.sub.2).fwdarw..infin. for (R.sub.1 +R.sub.2) .fwdarw.0.